docs: def export(self, nidm_version, export_dir):
          """
          Create prov graph.
          """
          self.stat = None
          if isinstance(self.stat_type, QualifiedName):
              stat = self.stat_type
          elif self.stat_type is not None:
              if self.stat_type.lower() == "t":
                  stat = STATO_TSTATISTIC
              elif self.stat_type.lower() == "z":
                  stat = STATO_ZSTATISTIC
              elif self.stat_type.lower() == "f":
                  stat = STATO_FSTATISTIC
              elif self.stat_type.startswith('http'):
                  stat = Identifier(self.stat_type)
  
          self.add_attributes((
              (PROV['type'], STATO_CONTRAST_WEIGHT_MATRIX),
              (NIDM_STATISTIC_TYPE, stat),
              (PROV['label'], self.label),
              (NIDM_CONTRAST_NAME, self.contrast_name),
              (PROV['value'], json.dumps(self.contrast_weights))))